<!DOCTYPE html>
<html lang="en">
<head>
    <meta charset="UTF-8">
    <meta name="viewport" content="width=device-width, initial-scale=1.0">
    <title>demo1</title>
</head>
<body>
    <!-- 标题标签<h+数字> -->
    <h1>hello h1</h1>
    <h2>hello h2</h2>
    <h3>hello h3</h3>
</body>
</html>